An image display device is proposed which uses a light emitting diode (LED) as a light emitting element. This type of the image display device includes a structure which includes: a plurality of LEDs which emit lights with each having a red (R), green (G) and blue (B) colored component; an illumination optical system in which lights emitted from the plurality of LEDs are incident; a light valve including a liquid crystal display panel on which lights emitted from the illumination optical system are incident; a color synthesizing prism which synthesizes the lights with each color emitted from the light valve; and a projection optical system for projecting lights emitted from the color synthesizing prism onto a projection surface. A semiconductor is used for the LED, and a semiconductor of groups III to V is usually used.
In order to enhance the luminance of a projected image, the requirement for an image display device that includes the above described structure is that, to the extent possible, it not generates optical loss in a light path from the light emitting element to the projection optical system.
Among the above described structural components, the liquid crystal display panel and the color synthesizing prism are components having polarization dependency, and accordingly it is desirable that the light emitting element emit linearly polarized light, in order to give the optical system higher efficiency.
In addition, as is described in Non Patent Literature 1 (SID 06 DIGEST, 2006, pp. 1808-1811, 61.1, Photonic Lattice LEDs for RPTV Light Engines, Christian Hoepiher), the efficiency of the optical system is restricted due to the etendue (Etendue) which is determined by a product of an area of the light emitting element and the radiation angle. Specifically, if the value of a product of the area of the light emitting element and the radiation angle is not controlled to be equal or less than the value of a product of an area of an entrance surface of the light valve and a light acceptance angle which is determined by an F number of the illumination optical system, the light emitted from the light emitting element will not be used as projection light.
For this reason, in the image display device using the LED, what is pending is to find a way to enhance the luminance of the light emitting element without enlarging the area of the light emitting element, in order to reduce the etendue of the light emitted from the light emitting element.
For the purpose of emitting a light having a large polarization ratio, Patent Literature 1 (JP2009-111012A) discloses a semiconductor light emitting element which has been prepared so that a nonpolar surface becomes the principal surface of crystal growth.
For the purpose of reducing the etendue and emitting linearly polarized light with high polarization conversion efficiency, Patent Literature 2 (JP 2007-109689A) discloses a light emitting element including: a light emitting portion which is provided on a reference plane and emits light; a λ/4 phase plate provided in an emission surface side of the light-emitting portion; a reflection-type polarizing plate which is provided in the emission surface side of the λ/4 phase plate, allows polarized light in a first vibration direction to pass therethrough and reflects polarized light in a second vibration direction which is perpendicular to the first vibration direction; and an optical portion which is provided in the emission surface side of the reflection-type polarizing plate and in which the refractive index periodically changes in two directions within a plane that is parallel to the reference plane.